I did a little internet exploring and found two news paper articles regarding the dropping of supplies. Neither article identified the plane used. Also found a site that stated that Lippiatt flew WACOs and Travel Airs almost exclusively. He did own a Lockheed Vega in the 30’s. How did you determine he used a Travel Air?
You might to try researching the Aviation Week archives. You must have a subscription to do a detailed online search but they might help.